This invention relates generally to vibration suppression and more particularly has reference to the control of electromechanical vibration suppressors to provide the proper electrical termination automatically in the case of a vibratory force having as a component a quasi-line in the spectrum which changes frequency slowly with time, to provide a termination that is optimum at the frequency of suppression and yet prevents or strongly moderates spurious resonances, and to provide optimum electrical termination over a moderately broad range of frequencies by a relatively simple negative impedance circuit.
This invention is an improvement on my previous invention disclosed in U.S. Pat. No. 3,088,062, for Electromechanical Vibratory Force Suppressor and Indicator, the disclosure of which is incorporated herein by reference. The specification for that patent describes a mechanical circuit (usually a spring s and a mass m in parallel) and a transducer with a particular external electrical termination Z.sub.e. The electrical termination may be at a remote point. The transducer transforms the total electrical impedance composed of the blocked impedance Z.sub.T of the transducer in series with the external electrical impedance Z.sub.e into a virtual mechanical element whose impedance is Z.sub.u. This is shown in FIG. 1 (a copy of FIG. 1b of the patent) in an equivalent electrical circuit where F is the open circuit force of the noise source, Z.sub.i is the internal impedance of the noise source, Z.sub.b is the impedance of the base on which the noise source rests and G is the electromechanical coupling constant. The theoretical basis of FIG. 1 is demonstrated in the Analysis portion of the Patent, Cols. 4, 5, 6, 7, and was verified with the experimental set-up shown in FIG. 2 (Patent FIG. 1a) in which the vibration suppressor was mounted on a thin plate, supported on pillow blocks and which was driven by a ballistically suspended shaker unit. The pillow blocks were mounted on a heavy horizontal baseplate. The results are given in FIG. 3 (similar to Patent FIG. 18) in which the experimentally determined optimum external capacitances (o) at discrete frequencies are compared with the theoretical curve (----). In an experiment using the set-up of FIG. 2, the relative velocity of suppression was determined as a function of frequency, the external terminating capacitance at each frequency being given by FIG. 3. The results are plotted in FIG. 4 (similar to Patent FIG. 17). If, on the other hand, a fixed external termination capacitor is used regardless of frequency, a greatly reduced velocity may be achieved at the frequency for which the given capacitor is the optimum termination, but, as the frequency moves either up or down, the velocity increases and may become larger than for the velocity without a suppressor (or dynamic absorber). Results are plotted in FIG. 5 (Patent FIG. 16). The case without suppression is plotted (----) and the case with suppression is plotted (----).
The kind of results obtained in FIG. 4 on a frequency by frequency basis can be obtained simultaneously over as wide a band of frequency as that over which the correct electrical termination can be achieved. FIG. 6 (corresponding to Patent FIG. 13) gives a positive feedback circuit of the "series" type for obtaining broad-band electrical termination. The impedance in the grid circuit can be given by Z.sub.3. Then the input impedance, which is to be used as the electrical impedance termination for the transducer Z.sub.e is given by EQU Z.sub.e =Z.sub.3 (1-A)+R.sub.o)
where A is the gain of the two stage amplifier (with negative feedback for stability), and R.sub.o its output impedance. The condition for the external electrical impedance for the parallel circuit in FIG. 1 to be resonant and thus make v.sub.b small is ##EQU1##
Then equating the right-hand sides of the last two equations gives the Z.sub.3 in FIG. 5 provided ##EQU2##